What might be the causative mechanism of this patient’s atrial fibrillation, based on his computed tomographic scan results? A man in his 50s with drug-resistant paroxysmal atrial fibrillation presented for pulmonary vein isolation. Transthoracic echocardiography demonstrated enlargement of the right atrium and ventricle with normal systolic cardiac function. Preprocedure cardiac computed tomographic angiography (Figure) showed a completely unroofed coronary sinus (URCS) with a right-sided superior vena cava (URSC type II), which was subsequently confirmed on transesophageal echocardiography. The patient underwent an ablation procedure in the left atrium with resolution of atrial fibrillation and symptoms. The URCS was not repaired.